* Western-backed draft threatens Syria with sanctions
* Russia says it will block the Western-backed draft
* US says Russian draft does not have enough support
By Michelle Nichols
UNITED NATIONS, July 16 (Reuters) - The U.N. Security Council will vote on Wednesday on a Western-backed resolution that threatens Syrian authorities with sanctions if they do not stop using heavy weapons in towns, despite a declaration by Russia that it will block the move.
The resolution, proposed by Britain, the United States, France and Germany, would extend a U.N. observer mission in Syria for 45 days and place international envoy Kofi Annan's peace plan under Chapter 7 of the U.N. Charter.
Chapter 7 allows the 15-member council to authorize actions ranging from diplomatic and economic sanctions to military intervention. U.S. officials have said they are talking about sanctions on Syria, not military intervention.
"Russia and China still expressed objection to Chapter 7, but when challenged they were unable to come up with any convincing reasons why," Britain's U.N. Ambassador Mark Lyall Grant told reporters on Monday after another round of talks.
"Obviously, we're happy to have further negotiations. We've scheduled a vote for Wednesday afternoon," he said.
According to the Western-backed resolution, Syria would face sanctions if it does not stop using heavy weapons and withdraw its troops from towns and cities within 10 days of the adoption of the resolution.
Russian Foreign Minister Sergei Lavrov said in Moscow on Monday that Russia would block the Western-backed resolution because of the threat of sanctions.
Russia and China have previously vetoed resolutions designed to pressure Syrian President Bashar al-Assad.
Russia, a key ally of Syria, has also put forward a resolution to extend the U.N. mission for 90 days, but it does not contain a threat of sanctions. It was not immediately clear when or if Russia planned to put its draft resolution to a vote.
"If our partners decide to block our resolution no matter what, then the U.N. mission will not have a mandate and will have to leave Syria. That would be a pity," said Lavrov, whose country is one of Syria's main arms suppliers.
The Security Council must decide the fate of the U.N. mission, known as UNSMIS, before its mandate expires on Friday.
When asked if the United States would block the Russian resolution, U.S. Ambassador to the United Nations Susan Rice said she did not believe the Russian draft resolution had enough support to pass in the first place.
Resolutions are legally binding and need nine votes in favor and no vetoes from the five permanent council members - the United States, Britain, France, Russia and China - to pass.
"We think it's very important that 18 months nearly into this conflict with over 100 people on average dying a day, the council do something different to change the dynamic on the ground," Rice said of the Western-backed resolution.
"The status quo is leading to intensified civil conflict with real potential to spill over in the region," she said.
China's U.N. Ambassador Li Baodong said Beijing believed it was important to show support for Annan's peace plan, but that there needed to be a "unified voice from the Security Council."
"We have problems with Chapter 7 elements, but we think that the council members should continue consultations to solve the differences," Li told reporters.
If the U.N. mission in Syria is renewed, U.N. chief Ban Ki-moon has recommended shifting the emphasis of the work of UNSMIS from military observers to civilian staff focusing on a political solution and issues including human rights.
UNSMIS suspended most of its monitoring activity on June 16 due to increased risk from rising violence.
While the mandate for 300 unarmed military observers is likely to be unchanged, diplomats said they have been told that only half that number would be required for the suggested shift in focus of the mission. The others would return to their home countries, but be ready to redeploy again at short notice. (Editing by Todd Eastham)